1. Field of Invention
The present invention relates to radioactive well logging techniques to measure the lateral flow of fluid in subsurface earth formations.
2. Description of Prior Art
In secondary and tertiary recovery of petroleum deposits many of the recovery techniques employ the injection of water or chemical solutions into the earth formations comprising the reservoir. In planning the recovery operation, the injection of water or chemical has in the past been limited by certain assumptions and/or approximations concerning the mobility of fluids in the formation comprising the reservoir. A crucial factor in such fluid injection programs is the vertical conformity of the producing formation as well as its horizontal permeability and uniformity. In some reservoirs, formation lensing or horizontal partitioning by permeability barriers such as faults can occur. In such instances, apparently correlevant intervals of permeability may be separated from one well to another in the field by such formation lensing or permeability barriers being interposed across the interval of formation between the wells.
It is therefore apparent that large amounts of costly chemicals or water could be injected before it is established that continuity between injection and producing wells is partially or totally absent. If, on the other hand, continuity does exist, it is known that the water in the vicinity of the producing well begins to flow laterally soon after injection is initiated and long before the injected fluid actually arrives at the producing well. The detection of lateral water flow is, therefore, indicative of formation continuity. The speed of the flow, when combined with injection rates, formation thickness, formation porosity, and well spacings, can be used to determine the degree of continuity. This early definition of formation continuity could prevent the expenditure of large sums of money, time and effort in a fruitless project to recover secondary or tertiary problem deposits.
A second application of the detection of lateral water-flow is the mapping of the total flow throughout a petroleum reservoir to help in the operational planning of injecting chemicals or water and to assist in determining optimum withdrawal rates. Moreover, a knowledge of the lateral water flow characteristics of a particular formation in a producing field can help greatly in general understanding of the reservoir dynamics of the particular reservoir being produced.
It is sometime desirable in a reservoir with multiple producing intervals for a reservoir engineer to be able to delineate those producing zones which provide the most water influx or water drive to the production of petroleum. The mapping of lateral water movement in all zones both above and below the expected water table in the producing formation should supply this information to the reservoir engineer.
In the past, reservoir engineers have been provided with relatively few and often inaccurate well logging instrumentation in order to determine the vertical conformance characteristics of the earth formations comprising a reservoir. This has led to resultant confusion as to the properties of the earth formations comprising a reservoir. Radioactive tracer studies of the movement of fluids in the vicinity of a well borehole can be misleading in this respect because of the lack of uniform absorption of the tracer element into the flowing stream of formation water. Also, it is difficult to provide tracer isotopes with sufficient half life to be injected at an injection well and observe their movement days or even weeks later at a monitoring or producing well, in order to obtain some idea of the lateral flow speed or velocity of fluids in the formation comprising the reservoir.
In co-pending application Ser. No. 698,934 referenced above, liquids in the formation adjacent the well were bombarded with high energy neutrons. Where the fluid was at least partially saline, as in salt water, radioactive Na.sup.24 was produced by the thermal neutron capture Na.sup.23 (n, .gamma.) Na.sup.24 reaction. By observing the decrease in gamma radiation from the Na.sup.24 at 2.75 MeV or greater energy levels with time, a measure of the horizontal speed of the liquid was obtained.
However, although steps were taken to remove neutron bombardment induced gamma radiation from other elements and normal background gamma radiation from earth formations, problems still existed. For example, manganese present as Mn.sup.55 in the steel well bore casing forms the radioactive isotope Mn.sup.56 upon neutron bombardment. Mn.sup.56 emits gamma radiation at several energy levels ranging from 0.85 MeV to 3.0 MeV and having a half-life of 2.56 hours. This gamma radiation has been found, according to the present invention, to contribute to observed counting rates of the Na.sup.24 gamma radiation. Although the Mn.sup.56 gamma radiation decays more rapidly than Na.sup.24 to a negligible level due to its shorter half-life, elimination of this component by time delay has presented problems, particularly where linear flow speed was relatively high or salinity relatively low, or both. Time delay in these circumstances resulted in loss of significant counting statistics in the Na.sup.24 gamma radiation as well as that of Mn.sup.56.